Atrial fibrillation (hereinafter referred to as “AF”) is the most frequently observed type of arrhythmia in clinical examinations. Although not a lethal arrhythmia, AF causes cardiogenic cerebral embolism, and is therefore recognized as an arrhythmia that greatly affects vital prognoses and QOL. It is known that the onset of AF increases with age, and that repeated AF strokes lead to chronic (serious) AF (The Journal of American Medical Association, 285, 2370-2375 (2001) and Circulation, 114, 119-123 (2006)).
To prevent chronic AF, which causes difficulty in restoring sinus rhythm and increases the risk of cardiogenic cerebral embolism, early defibrillation and subsequent prevention of recurrence (maintenance of the sinus rhythm) are required. Antiarrhythmic drugs (classes I and III) are most commonly used as pharmacotherapy, but these drugs achieve insufficient therapeutic effects, while causing serious side effects such as a proarrhythmic effect (Am. J. Cardiol., 72, B44-B49 (1993)).
The onset of AF is triggered by atrial premature contraction with underlining causes such as intra-atrial conduction delay, shortening and heterogeneity of the atrial refractory period (Nature Reviews DRUG DISCOVERY 4, 899-910 (2005)). It is known that the prolongation of refractory period of atrial muscle can terminate AF (defibrillation) or prevent the occurence of AF. The action potential duration of the mammalian cardiac muscle is predominantly determined by voltage-dependent K+ channels. Inhibition of the K+ channel prolongs myocardial action potential duration, which results in prolongation of the refractory period (Nature Reviews DRUG DISCOVERY 5, 1034-49 (2006)). The action mechanism of class III antiarrhythmic drugs (e.g., Dofetilide) is to inhibit rapid delayed rectifier K+ current (IKr), K+ current encoded by HERG. However, since IKr is present in both the atria and ventricles, such drugs might cause ventricular arrhythmias, such as torsades de pointes (Trends Pharmacol. soc., 22, 240-246 (2001)).
Ultra-rapid delayed rectifier K+ current (IKur), K+ current encoded by Kv1.5, has been identified as K+ channel that is specifically expressed only in human atria (Cric. Res., 73, 1061-1076 (1993), J. Physiol., 491, 31-50 (1996) and Cric. Res., 80, 572-579 (1997)). Muscarine potassium current (IKACh) encoded by two genes called GIRK1 and GIRK4 is known as a K+ channel specifically expressed in human atria (Nature 374, 135-141 (1995)). Accordingly, a pharmacologically acceptable substance that selectively blocks the IKur current (the Kv1.5 channel) or the IKAch current (GIRK1/4 channel) can act selectively on the atrial muscle and is considered effective to exclude the proarrhythmic effect caused by prolonged action potential duration of the ventricular muscle.